This invention is directed to a keying system for electronic musical instruments and more particularly to a time division multiplexed keying system for an electronic organ.
The synthesis electronic organ is based upon the knowledge that sustained complex musical tones can be synthesized by mixing properly scaled sine waves having frequencies representative of the fundamental and the various harmonics of the tones to be synthesized. This is done by having each organ playing key operate a group of contacts such that when the playing key is pressed it connects sine wave generators corresponding to the various frequencies required to a corresponding group of busses. By connecting the various busses to the output through voltage divider resistors or transformer primary taps the relative strength of the various harmonics is adjusted as desired. Usually nine separate contacts for each of the playing keys are necessary in such synthesis organs. This multiple contact keying system is referred to as alternating current (AC) keying.
The multiple contacts of the individual playing keys in the synthesis electronic organ create various problems. For example, they tend to be somewhat stiff and accordingly present a greater mechanical resistance to the depression of an individual key than is desirable for some organists. Also, AC keying requires that low level signals be passed directly through each contact and, accordingly, the contacts must be of a very high quality and hence expensive. Keying transients are difficult to control and there are also limitations on control of the keying envelope.
Due to the problems encountered with AC keying, direct current (DC) keying has been developed. In DC keying of a synthesis organ, a single contact key passes a direct current signal to control a plurality of transistor switches. These switches connect the various frequencies required for a given tone to filter circuits which are in turn connected to audio output circuits to sound the desired tone. An example of this type of DC keying system is disclosed in U.S. Pat. No. 3,636,231.
In the interest of simplifying electronic organ circuits and reducing the amount of interconnections and circuitry required therein, various time division multiplexing arrangements have been developed. For example, in one system the various control signals from the draw bars and tabs are multiplexed and applied in multiplex form to a DC keyer while each group of signals from the keyboard manuals are multiplexed and applied to the DC keyer in synchronism with the multiplexed control signals. This known time multiplexed arrangement receives the various tone signal inputs unmultiplexed, applied multiplexed signals from the draw bars and keyboards to generate time multiplexed output signals which are correspondingly demultiplexed and applied to output filter circuits. Still other systems provide for multiplexing keydown signals from the organ keyboard to eliminate the wiring otherwise required to connect the keyboard to the tone generating and sounding circuitry.